A secondary battery such as a lithium ion battery generally includes members such as a positive electrode, a negative electrode and a separator.
The separator typically includes a resin microporous film and is disposed between a positive electrode and a negative electrode. The separator plays a role of separating the positive electrode and the negative electrode to prevent internal short circuit due to contact between both electrodes, and a role of allowing ions such as lithium ions to pass therethrough to enable discharge and charge of the secondary battery.
Conventionally, as a separator including a resin microporous film, a microporous film containing polyethylene having a specific weight average molecular weight and polypropylene having a specific weight average molecular weight, and a separator including this microporous film (Patent Literature 1), a microporous film obtained by using a polyethylene composition containing an ultrahigh molecular weight polyethylene and a low density polyethylene, and a separator including this microporous film (Patent Literature 2), and the like are known.
The separators described in these literatures contain two or more polymer components. In addition, when the temperature in the battery abnormally increases, a polymer having a lower heat distortion temperature melts to close the micropores, and a polymer having a higher heat distortion temperature holds the structure of the whole separator. These actions of each polymer can suppress movement of ions in the secondary battery to prevent ignition or the like of the secondary battery even when the temperature in the battery abnormally increases.
When ignition or the like of the secondary battery is prevented by such a mechanism, it is desirable that the temperature at which the whole separator melts and deforms (hereinafter referred to as “meltdown temperature” in some cases) is higher.